The customary steps of primary and secondary reforming to produce ammonia synthesis gases and primary reforming to produce methanol synthesis gases are well known both technically and economically. From the latter view point, these steps are recognized as controlling factors in determining the "feed and fuel" requirements for each unit of ammonia or methanol produced because both steps require heat from combustion of hydrocarbon for the endothermic reaction of steam with hydrocarbon feed.
Commercial primary reformers are fuel fired furnaces having large tubes filled with nickel-containing catalyst wherein approximately 60 to 80 volume percent of the fresh hydrocarbon feed is converted with added steam to hydrogen and carbon oxides. This primary reformed gas additionally contains unreacted steam and the balance of the hydrocarbon feed as methane. From the process viewpoint, the primary reformer is an endothermic catalytic steam reforming zone.
For ammonia production the primary reformed gas then is introduced to the secondary reformer which is typically a refractory-lined vessel filled with nickel-containing catalyst and has no provision for supply of external heat. In secondary reforming, heat for endothermic reaction of the remaining methane with steam is supplied by combustion of part of the primary reformed gas with externally supplied oxidant. From the process viewpoint, the secondary reformer is an exothermic catalytic steam reforming zone and is sometimes referred to as an autothermal reformer.
The hot synthesis gas produced in secondary reformer is comprised of hydrogen, nitrogen, carbon monoxide (which is subsequently converted to additional hydrogen), carbon dioxide, unreacted steam, residual methane, and small quantities of inert gases. Customarily, this hot synthesis gas is heat exchanged with boiler feedwater to raise turbine steam required in compression services for secondary reformer oxidant, synthesis gas, and refrigerant employed in ammonia or methanol product recovery.
Despite this use, practitioners have long desired to employ heat of the secondary reformer outlet gas in the alternative service of primary reforming through use of a reactor/heat exchanger and thereby minimize size of the conventional fired tube reforming furnace. Ideally, the furnace would be deleted if sufficient primary reforming duty could be moved to the secondary reformer in order to balance heat requirement of the endothermic reforming step with heat availability from the exothermic reforming step. This heat balance requires substantially more combustion in the secondary reformer, hence use of excess oxidant. In the production of ammonia synthesis gas where air is employed as an oxidant the need for an excess of this oxidant necessitates downstream removal of excess nitrogen to achieve the desire hydrogen/nitrogen ratio in the final ammonia synthesis gas.
Reactor/exchangers proposed for this service have been high temperature heat exchangers having single-pass tubes fixed at each end to tube sheets. While considerably less costly than fired tube furnaces, their high temperature design leads to high fabrication cost. Perhaps more importantly, particularly in the production of ammonia synthesis gas, is the large quantity of excess nitrogen in the final synthesis gas which results from the heat balance problem indicated above which leads to the necessity for an uneconomically large nitrogen rejection system preceding or within the synthesis section of an ammonia plant.
More recently, open-end bayonet tube reactor/exchangers of the general type shown in U.S. Pat. No. 2,579,843 have been considered for primary reforming service because of their more simple design in comparison with single-pass exchangers. In already known designs for the production of ammonia and methanol synthesis gases which employ open-end bayonet tubes, the heat balance problem mentioned above with regard to production of ammonia synthesis gas has precluded elimination of the conventional fired tube reforming furnace.
It is, therefore, an object of this invention to produce synthesis gases for use in the production of ammonia and methanol and to utilize heat from exothermic catalytic reforming in the endothermic reforming step under such conditions that the entire heat of conversion is furnished from the exothermic reforming step.